doc: fix link to Planck paper

docs/co_lines.rst

@@ -3,7 +3,7 @@
 COLines
 =======
 
-This class implements simulations for Galactic CO emission involving the first 3 CO rotational lines, i.e. :math:`J=1-0,2-1,3-2` whose center frequency is respectively at :math:`\nu_0 = 115.3, 230.5,345.8` GHz. The CO emission map templates are the CO Planck maps obtained with ``MILCA`` component separation algorithm (See `Planck paper <https://www.aanda.org/articles/aa/abs/2014/11/aa21553-13/aa21553-13.html>`). The CO maps have been released at the nominal resolution (10 and 5 arcminutes). However, to reduce  noise contamination from template maps (especially at intermediate and high Galactic latitudes), we  convolved them with a 1 deg gaussian beam.
+This class implements simulations for Galactic CO emission involving the first 3 CO rotational lines, i.e. :math:`J=1-0,2-1,3-2` whose center frequency is respectively at :math:`\nu_0 = 115.3, 230.5,345.8` GHz. The CO emission map templates are the CO Planck maps obtained with ``MILCA`` component separation algorithm (See `Planck paper <https://www.aanda.org/articles/aa/abs/2014/11/aa21553-13/aa21553-13.html>`_). The CO maps have been released at the nominal resolution (10 and 5 arcminutes). However, to reduce  noise contamination from template maps (especially at intermediate and high Galactic latitudes), we  convolved them with a 1 deg gaussian beam.
 
 The Stokes I map is computed from the template one as it follows: